The present invention relates to a display control apparatus for displaying data having gradations on a display unit such as a CRT (Cathode Ray Tube) or the like.
In recent years, in order to display various kinds of information such as channels, volumes, and the like as characters or graphics on a display unit such as a CRT or the like, a display control apparatus is employed in a television set or the like. This display control apparatus reads display data stored in a ROM in order of display codes stored in a RAM, converts the display data into RGB display data, further converts the RGB display data into analog RGB data having halftones by a DAC (Digital Analog Converter), and displays the analog RGB data on a display unit.
FIG. 3 is a block diagram illustrating a display control apparatus using a conventional technique. A main unit 300 of the display control apparatus displays display data stored in it, on a display unit 110 such as a CRT, on the basis of a display command S101 supplied from the outside. The display command S101, which is a signal instructing to carry out display, is input to a display command input terminal 101. A CPU (Central Processing Unit) 102 generates a display code S102 on the basis of the display command S101 inputted to the display command input terminal 101. The display code S102 is a signal corresponding to display data to be displayed. For example, when displaying a character on the display unit 110, an ASCII code or the like is used as the display code, and the display data corresponding to this display code is bit map data of the character, or the like. The display codes S102 generated by the CPU 102 are sequentially arranged and stored in a RAM 103. An address generation controller 104 sequentially reads the display codes S103 arranged in the RAM 103 and performs arithmetic operation on them to convert the display data into ROM addresses S104 of a ROM 105 which stores the display data. The ROM 105 outputs the display data S105 at an address position on the basis of a given ROM address S104. The display data S105 is a 5-bit signal, and it is used to specify 27 display colors to be converted in a color data conversion unit 306. In the color data conversion unit 306, a code of a color pallet used to control a DAC 207 is stored. The color data conversion unit 306 converts the 5-bit display data S105 for specifying the display colors into display color control signals S306a, S306b, and S306c for specifying the display colors of R(Red), G(Green), and B(blue) each comprising 2 bits, on the basis of the color pallet, and outputs these signals. A display control preprocessing unit 350 is composed of the CPU 102, the RAM 103, the address generation controller 104, the ROM 105, and the color data converter 106.
The DAC 207 is a current output type DAC receiving the display color control signals S306a, S306b, and S306c corresponding to R, G, and B each having 2 bits, which are output from the display of the display control preprocessing unit 350. The DAC 207 comprises a DACR 217 corresponding R, a DACG 227 corresponding to G, and a DACB 237 corresponding to B, and grounding resistors RR3, RG3, and RB3 which are connected to the outputs of the DACR 217, the DACG 227, and the DACB 237, respectively, to convert current into voltage. The DACR 217, the DACG 227, and the DACB 237 perform control of three current outputs of H (high) level, M (Middle) level, and L (low) level, on the basis of the values of the 2-bit display color control signals S306a, S306b, and S306c, respectively, and the current outputs are converted into three voltage levels of H, M, and L, thereby outputting display color data S107a, S107b, and S107c that can be displayed by the display unit 108.
In the display control apparatus 300 so constructed, when each of the three display color data S107a, S107b, S107c represents the three signal levels, 27 (=3xc3x973xc3x973) states are generated, whereby 27 colors can be displayed. Each of the current output type DACs, i.e., the DACR 217, DACG 227, and DACB 237, is able to control the current outputs of three levels H, M, L, according to the inputted 2-bit display color control signal and, therefore, 27 (=3xc3x973xc3x973) voltage states are realized by the resistors RR3, RG3, and RB3 that convert current into voltage.
However, the conventional display control apparatus 300 employs the current output type DACs that are analog circuits. Generally, in contrast with a digital circuit, the characteristics of an analog circuit greatly vary according to the size of a transistor or the capacitance of a capacitor, which are constituents of the analog circuit. Therefore, the manufacturing precision must be kept high to obtain products of uniform quality. Further, the power consumption is increased. Consequently, the conventional display control apparatus has the problems of high manufacturing cost and large power consumption.
The present invention is made to solve the above-described problems and has for its object to provide a display control apparatus that can perform intermediate color display, with reduced manufacturing cost and reduced power consumption.
Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.
According to a first aspect of the present invention, there is provided a display control apparatus including at least one output control unit, and each output control unit comprises: a tri-state buffer receiving, as a signal input, a one-bit color signal, and receiving, as a control input, a one-bit intermediate color control signal for controlling an intermediate color of the color signal, and setting its output in one of the following three states: a state of outputting a first voltage, a state of outputting a second voltage, and a high impedance state, on the basis of the color signal and the intermediate color control signal; a first resistor having an end connected to a power supply, and the other end connected to the output of the tri-state buffer; and a second resistor having an end connected to the ground, and the other end connected to the output of the tri-state buffer. Therefore, it is possible to provide a display control apparatus that can perform output for intermediate color display, with reduced manufacturing cost and reduced power consumption, by using the tri-state buffer as a digital circuit.
According to a second aspect of the present invention, the display control apparatus of the first aspect includes, as the output control units, an output control unit to which an R color signal is input, an output signal unit to which a G color signal is input, and an output control unit to which a B color signal is input. Therefore, it is possible to provide a display control apparatus that can perform output for intermediate color display, with reduced manufacturing cost and reduced power consumption.
According to a third aspect of the present invention, the display control apparatus of the first aspect including a plurality of the output control units, and the respective output control units receive intermediate color control signals which are independent of one another. Therefore, it is possible to provide a display control apparatus that can perform output for intermediate color display, with reduced manufacturing cost and reduced power consumption.
According to a fourth aspect of the present invention, in the display control apparatus of the first aspect, the output control unit further includes a switch between the power supply and the first resistor, which switch electrically connects the power supply and the first resistor when the color signal and the intermediate color control signal are input to the output control unit, and electrically disconnects the power supply and the first resistor when the color signal and the intermediate color control signal are not input to the output control unit. Therefore, it is possible to provide a display control apparatus that can reduce unnecessary power consumption.
According to a fifth aspect of the present invention, in the display control apparatus of the first aspect, the output control unit further includes a switch between the ground and the second resistor, which switch electrically connects the ground and the second resistor when the color signal and the intermediate color control signal are input to the output control unit, and electrically disconnects the ground and the second resistor when the color signal and the intermediate color control signal are not input to the output control unit. Therefore, it is possible to provide a display control apparatus that can reduce unnecessary power consumption.